Sulfuric acid mist is a significant problem downstream of a coal or oil-fired boiler. It often forms as a sub-micron mist when the hot flue gas is cooled to the sulfuric acid dew point by mixing with cooler air in the atmosphere or with an aqueous stream or spray in a scrubbing system. The production of sulfur trioxide (SO.sub.3) and resulting sulfuric acid mist is proportional to the amount of sulfur in the coal or oil and is a particular problem if oxidation catalysts such as vanadium and nickel are present in the flue gas. The acid mist often condenses in cool corners of electrostatic precipitators, ducts or bag houses causing corrosion. It also forms when the hot gas stream is quenched by liquid or slurry spray at the inlet to a sulfur dioxide scrubber. In such scrubbing systems, most of the sulfuric acid mist normally passes through the scrubber without being removed. Sulfuric acid plumes are common downstream of the stack on many oil-fired boilers. These plumes result from the cooling of the hot stack gas in the cooler air in the atmosphere. The fine mist is a form of visible pollution and results in acid rain down-wind of the plant.
Hydrochloric acid and hydrofluoric acid are acid gases generated while firing coal. These acids are not so easily condensed as sulfuric acid, but are very soluble in aqueous solutions or in rain droplets. Many sulfur dioxide scrubbing systems are plagued with high concentrations of chlorides and/or fluorides that build up in scrubbing liquors due to their removal from the flue gas. The chlorides in the solutions reduce the reactivity of limestone in sulfur dioxide scrubbing systems. Chlorides and fluorides cause corrosion problems with sulfur dioxide removal equipment. They also contaminate by-products such as gypsum that are often produced from such sulfur dioxide removal processes. Removal of these acid gases upstream of the scrubbers would eliminate these corrosion and contamination problems and improve the reactivity of the limestone used to neutralize and precipitate the absorbed sulfur dioxide.
One method of removal of sulfuric (H.sub.2 SO.sub.4 or SO.sub.3 in gas form) hydrochloric (HCL), and hydrofluoric (HF) acids from stack gases is to inject lime or soda ash into the gas stream either in a duct or in a spray dryer. U.S. Pat. No. 4,865,828 dated Sep. 12, 1989--Removal of Acid Gases in Dry Scrubbing of Hot Gases, Lerner, Bernard J., relates to such a method. This patent describes basic solutions, such as sodium carbonate, that are used to remove acid gases from flue gases with the primary emphasis on sulfur dioxide (SO.sub.2) removal. Other methods are similar to the Lerner patent in that solutions of alkaline salts are injected into flue gas streams to remove sulfur dioxide and other acid gases simultaneously. However, existing processes do not differentiate between the gases. Consequently, in cases where it is desirable technically or economically, to remove HCl, HF and SO.sub.3 prior to sulfur dioxide scrubbers, or to remove only HCl, HF and SO.sub.3, the existing processes offer no solution.
It is thus desired in the art to provide a method for removing acid gases such as sulfuric, hydrochloric and hydrofluoric acids from flue gas streams, and that such removal be separate from sulfur dioxide removal.